OBJECTIVE: To elucidate the control mechanisms regulating the commitment of hemopoietic stem cells and maturation of early hemopoietic precursors. We are developing a long-term suspension culture system for the sustained production of early human hemopoietic precursors. At the present time, we are able to maintain erythroid precursors for two to three weeks and granuloid precursors for four to six weeks. This system will be used to study the commitment of hemopoietic stem cells to monopotent precursors such as BFU-e and CFU-c. The role of the marrow stroma in the hemopoietic inductive microenvironment is being investigated using implants of deminerlized bone matrix. Ossicles containing histologically recognizable hemopoietic tissue develops in a few weeks. In vitro analysis of the ossicle marrow carried out in a methylcellulose culture system demonstrated the presence of committed hemopoietic precursors (CFU-c, CFU-e and BFU-e) by six weeks post-implantation. Erythroid burst-promoting activity (BPA) has been found in human bone marrow suspension cultures. Further characterization of this factor and its role in erythroid development are being carried out. We hope to be able to ascertain the cell type responsible for its production and the target cell which responds to BPA.